Radiation diffractive materials based on crystalline colloidal arrays have been used for a variety of purposes. A crystalline colloidal array (CCA) is a three-dimensional ordered array of monodispersed colloidal particles. The particles are typically composed of a polymer latex such as polystyrene or an inorganic material, such as silica.
Such colloidal dispersions of particles can form crystalline structures having lattice spacings that are comparable to the wavelength of ultraviolet, visible or infrared radiation. These crystalline structures have been used for filtering narrow bands of selected wavelengths from a broad spectrum of incident radiation, while permitting the transmission of adjacent wavelengths of radiation. Alternatively, CCAs are fabricated to diffract radiation for use as colorants, markers, optical switches, optical limiters and sensors.
Many of these devices have been created by dispersing particles in a liquid medium, whereby the particles self-align into an ordered array. The particles are fused together by mutual polymerization or by introducing a solvent that swells and fuses the particles together.
Recently, other devices have been produced from hydrogels containing CCAs polymerized within the hydrogel. In hydrogel-based devices, similarly charged colloidal particles are dispersed in a low-ionic strength liquid media. The particles self-assemble into a CCA due to their electrostatic charges. These ordered structures diffract radiation according to Bragg's law, wherein the radiation meeting the Bragg conditions are reflected, while adjacent spectral regions that do not meet the Bragg conditions are transmitted through the device.
Other CCAs are produced from a dispersion of monodispersed particles bearing a similar charge in a carrier. The dispersion is applied to a substrate, and the carrier is evaporated to yield an ordered periodic array of the particles. The array is fixed in place by coating the array with a curable polymer. The fixed array, which diffracts radiation according to Bragg's law, may be used in place or may be removed from the substrate in the form of a film or as flakes.
An ordered periodic array of particles that diffracts radiation according to Bragg's law satisfies the equation:mλ=2nd sin θwhere m is an integer, λ is the wavelength of reflected radiation, and n is the effective refractive index of the array, d is the distance between the layers of particles, and θ is the angle that the reflected radiation makes with the plane of a layer of particles. Incident radiation is partly reflected at an uppermost layer of particles in the array at angle θ to the plane of the first layer and is partially transmitted to underlying layers of the particles. While some absorption incident radiation occurs as well, a portion of the transmitted radiation is partially reflected at the second layer of particles in the array at angle θ and partially transmitted to the underlying layers of particles. This feature of partial reflection at angle θ and partial transmission to the underlying layers of particles continues through the thickness of the array. The wavelength (λ) of diffracted radiation can be controlled by the dimension (d), which may be the distance between the planes of the centers of the particles in each layer. Generally, the diffracted wavelength (λ) is proportional to the particle diameter (d) for an array of packed particles. Thus, the interparticle interaction is a factor in producing CCAs.